hey Improvise ,
Could be a few things , including stuffed wastegate spring , but most common is how it's(the boost controller that is) plumbed up.
Make sure that there is nothing else inline with the boost control.ie: hose straight from the pressure side of the inlet to the boost control ,to the wastegate.
Block off the factory boost soleniod & unplug the electrical connector.
Having said that , all bleed style boost controls will "spike" a little , especially in low gears
& most stock turbos aren"t that efficient high up in the rev range.
I use a "Gated" Turbosmart boost control , & have done for years , with no probs